1. Field of the Invention
The invention relates to a method for deriving a tracking error signal in an optical storage system, and more particularly, to a method for deriving a tracking error signal based on a first analog detection signal and a second analog detection signal.
2. Description of the Prior Art
An optical pick-up head, which is used to access data, plays an important role in various optical storage systems. Taking an optical drive as an example, the basic infrastructure of the optical drive can be seen in FIG. 1, which is a schematic diagram of a typical optical drive 10. The typical optical drive 10 includes a pick-up head 12, a spinning motor 14, and a movement stage 16. The pick-up head 12 is used to focus an optical beam 18 on a surface of a recording carrier 20 (an optical disk) to form an optical spot whose area is close to the data area of the optical disk 20. The spinning motor 14 rotates the optical disk 20.
Ideally, the optical spot propagates along a track direction on the optical disk 20 to form an optical spot trace 22 to access data on the optical disk 20. The pick-up head 12 is connected to the movement stage 16, and the movement stage 16 can assist the pick-up head 12 to seek tracks, so that the pick-up head 12 can move appropriately to a target track on the optical disk 20 to read or to write data. Taking a data-reading process as a brief example, after the optical beam emitted from the pick-up head 12 is reflected and refracted from an information plane of the optical disk 20, an optical sensor will receive the reflected (refracted) light. According to the different areas on the optical disk 20 respectively representing 0 and 1, the reflected light will show different optical intensities. The optical sensor will transform the reflected light of the different optical intensities into corresponding voltage signals.
When the optical drive 10 operates, the optical disk 20 is rotated at a very high frequency. Operating characteristics of the optical disk 20 in such circumstances are prone to be highly temperature-dependent and external-force-dependent. In addition, due to the optical disk 20 being a detachably installed recording carrier, the rotating center of the optical disk 20 may deviate from the predetermined center of rotation, so that the optical disk 20 may operate unstably, causing focus errors and tracking errors. Therefore, the pick-up head 12 is required to lock the optical spot along the desired data track on the optical disk 20 to accurately and quickly access data.
Moreover, the optical disk 20 shown in FIG. 1 is used to store high-density data, so that the width of the data tracks and distance between the data tracks are both very short. Therefore, any deviation from the data track will lead to incorrect data accessing. When being practically implemented, the optical spot emitted from the optical pick-up head should be perfectly located at the data track.
Please refer to FIG. 2, which is a schematic diagram showing a spatial relationship between a data track on the optical disk 20 shown in FIG. 1 and an optical sensor 30 of the pick-up head 12. A plurality of pits 32 of different lengths are strewn along the data track. An arrow mark 34 shown in FIG. 2 represents a track direction of the data track on the optical disk 20, and the optical sensor 30 of the pick-up head 12 moves and accesses data along the arrow mark 34 on the optical disk 20. The optical sensor 30 is a four-dimensional sensor, including a section A, a section B, a section C, and a section D. When each pit 32 on the data track passes through the optical sensor 30 of the pick-up head 12, the optical sensor 30 can be used to receive the optical beam 18 reflected and refracted from the pit 32. A tracking error signal TE and a focus error signal FE are generated according to four received different portions in space of the optical beam 18 respectively corresponding to the four sections (the section A, the section B, the section C, and the section D) of the optical sensor 30. The tracking error signal TE represents a deviation of the optical spot away from the data track. The focus error signal FE represents a distance between a focal point of the optical beam 18 shown in FIG. 1 and the information plane of the optical disk 20. According to the tracking error signal TE and the focus error signal FE, the position of the optical pick-up head 12 can be dynamically adjusted. Some prior art patents whose subject is aimed at generating the tracking error signal TE based on the above-mentioned optical sensor 30 are discussed below.
In U.S. Pat. No. 4,057,833, “Centering detection system for an apparatus for playing optically readable recording carriers”, Braat et al. utilize an allanalog technique to generate a plurality of corresponding output signals according to the plurality of portions of the optical beam. Braat et al. then make use of time differences or phase differences among those output signals to generate the tracking error signal TE.
For increasing the accuracy of generated signals, Bakx et al. teach an alldigital approach to process data in U.S. Pat. No. 6,137,755, “Deriving a tracking error signal from a time difference between detector signals”. Regarding the structure disclosed by Bakx et al, please refer to FIG. 3, which is a functional block diagram of a tracking error signal generator 40 according to the prior art. As shown in FIG. 3, the tracking error signal generator 40 includes two signal input ports (a first signal input port 42 and a second signal input port 44), two digitizers (a first digitizer 46 and a second digitizer 48), a digital delay device 50, two comparators (a first comparator 52 and a second comparator 54), and a signal generator 56. The first signal input port 42 is used to receive a first analog detection signal A1 and the second signal input port 44 is used to receive a second analog detection signal A2.
Please also refer to FIG. 2. The four detecting sections A, B, C, D of the optical sensor 30 can be used to respectively generate four corresponding output signals a, b, c, d, according to corresponding portions of the optical beam. If there is a deviation between the optical spot and the data track, there are time differences between the output signals a, b, c, d. For clarifying the degree of the deviation, a first analog detection signal A1 is set as a sum of the output signal a and the output signal c (A1=a+c), and a second analog detection signal A2 is set as a sum of the output signal b and the output signal d (A2=b+d). Please continue to refer to FIG. 3. The first signal input port 42 and the second signal input port 44, which are respectively connected to first digitizer 46 and second digitizer 48, are respectively used to transform the first analog detection signal A1 and the second analog detection signal A2 into a first digital detection signal D1 and a second digital detection signal D2.
Please refer to FIG. 4 that is a time sequence diagram showing variations of a plurality of signals generated in FIG. 3. As shown in FIG. 4, there is a time difference Δ between the first digital detection signal D1 and the second digital detection signal D2, and the time difference Δ represents the deviation between the optical spot and the data track.
Please return to FIG. 3. The digital delay device 50 is electrically connected to the first digitizer 46 for digitally delaying the first digital detection signal D1 with a delay time Td to generate a digital delay signal DR. Afterwards, the digital delay signal DR and the first digital detection signal D1 will pass through the first comparator 52 to generate a first digital comparing signal DC1. The first comparator 52 can be an XOR (Exclusive OR) logic gate and mainly used to extract front edges and rear edges of the digital delay signal DR and the first digital detection signal D1. Similarly, the digital delay signal DR and the second digital detection signal D2 pass through the second comparator 54 (an XOR logic gate) to generate a second digital comparing signal DC2, and the first digital comparing signal DC1 and the second digital comparing signal DC2 are both shown in FIG. 4. The first comparator 52 and the second comparator 54 are jointly connected to the signal generator 56. The signal generator 56 can be used to subtract the first digital comparing signal DC1 from the second digital comparing signal DC2 to generate a time-difference signal DT.
According to the time-difference signal DT, the related circuitry can discriminate the relationship between the first digital detection signal D1 and the second digital detection signal D2 in time domain. As shown in FIG. 4, the time-difference signal DT is a negative voltage value that represents that the first digital detection signal D1 transcends the second digital detection signal D2. Afterwards, the signal generator 56 can be used to process the time-difference signal DT to generate the tracking error signal TE. Therefore, the optical spot emitted from the optical pick-up head 12 shown in FIG. 1 can dynamically move along the track direction shown by the arrow mark 34 in FIG. 2 according to the tracking error signal TE.
Another structure disclosed in prior art patents is shown in FIG. 5, which is a functional block diagram of a tracking error signal generator 60. The difference between the embodiment of FIG. 5 and that of FIG. 3 is that the FIG. 5 embodiment includes four signal input ports 62. Without executing a signal combination process, the four signal input ports 62 directly and respectively receive the four corresponding output signals a, b, c, d generated by the four detecting sections A, B, C, D of the optical sensor 30 shown in FIG. 2. The four corresponding output signals a, b, c, d can be respectively treated as a first analog detection signal A1, a second analog detection signal A2, a third analog detection signal A3, and a fourth analog detection signal A4 in the tracking error signal generator 60. Except for the above-mentioned difference, all the other characteristics of this embodiment, including the all digital operations, are the same as in the previous one.
The tracking error signal generator 60 further includes four digitizers 64, which are respectively electrically connected to the four signal input ports 62 and are used for respectively transforming the first analog detection signal A1, the second analog detection signal A2, the third analog detection signal A3, and the fourth analog detection signal A4 into a first digital detection signal D1, a second digital detection signal D2, a third digital detection signal D3, and a fourth digital detection signal D4. The embodiment shown in FIG. 5 includes two first digitizers 70 used for respectively delaying the first digital detection signal D1 and third digital detection signal D3 into a first digital delay signal DR1 and a third digital delay signal DR3. Similar to FIG. 3, four comparators 68 (XOR logic gate) can be used. The first digital delay signal DR1 can be compared with the first digital detection signal D1 to generate a first digital comparing signal DC1. The first digital delay signal DR1 can also be compared with the second digital detection signal D2 to generate a second digital comparing signal DC2. The third digital delay signal DR3 can be compared with the third digital detection signal D3 to generate a third digital comparing signal DC3. In addition, the third digital delay signal DR3 can be compared with the fourth digital detection signal D4 to generate a four digital comparing signal DC4. Finally, a signal generator 66 will operate an adding/subtracting combination on the four digital comparing signals (in this embodiment, the adding/subtracting combination of the four digital comparing signals can be described as: DC2+DC4−DC1−DC3) to generate the tracking error signal TE.
Although the above-mentioned prior art structures and methods for generating the tracking error signal TE are widely used, there is still room for improvement. First of all, in the embodiment shown in FIG. 3, only the first digital detection signal D1 is delayed by the first digitizer 50, and the delayed first digital detection signal D1 is used as a comparing criterion for the first digital detection signal D1 and the second digital detection signal D2. That is, the above-mentioned prior art neglects to put the second digital detection signal D2 into consideration when operating related delaying and comparing operations. Therefore, in some specific circumstances, the above-mentioned neglect will lead to an imbalance effect between signals. The imbalance effect will be aggravated in the embodiment shown in FIG. 5.
In addition, when the optical drive changes its rotational speed, the frequency of an RF signal reproduced by the optical sensor 30 shown in FIG. 2 (the RF signal can be treated as a sum of the four output signals a, b, c, d shown in FIG. 2) will be correspondently changed, and the delay time of the first digitizer (50, 70) should be correspondently adjusted. Therefore, both the first digitizer 46 shown in FIG. 3 and the first digitizer 64 shown in FIG. 5 should be externally or internally installed with a tuning circuit to adjust the delay time according to the frequency of the RF signal. The installation of the tuning circuit, which is difficult for digital circuitry and is likely to increase the area of the first digitizer, is a great burden for the tracking error signal generator and the whole optical storage system.